The present invention relates, in general, to a security document, such as a banknote, for example.
Nothing in the following discussion of the state of the art is to be construed as an admission of prior art.
Security documents of a type involved here include an optical security area and a security sign which is also known as “hidden images” and is normally difficult to recognize by the naked eye. Examples of security signs include hidden words, images or geometrical figures. Such security areas are often directly printed onto the security document and thus can be counterfeit in a relatively simple way by using high-resolution copying machines, or can also be integrated in the security document by professional counterfeiters in a relatively simple manner by scanning and subsequent image processing.
It would therefore be desirable and advantageous to provide an improved value document to obviate prior art shortcomings and to show a higher level of security against counterfeiting.